1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device provided with a voltage converter circuit for supplying a driving voltage to an integrated circuit.
2. Description of the Prior Art
Recently, it has been proposed to reduce the driving voltage supplied to an integrated circuit, as far as the integrated circuit can operate normally, to decrease the power consumption of the integrated circuit.
FIG. 20 is a block diagram showing a prior art voltage converter circuit for converting a supply voltage supplied from the outside to a lower driving voltage of an integrated circuit, and FIG. 21 is a diagram showing the operating waveforms thereof at various points of the voltage converter.
A clock signal A as shown by (a) in FIG. 21 is applied to an input terminal of the voltage converter circuit shown in FIG. 20, and then inverted by an inverter 121. The inverted clock signal is given to one input terminal of a NAND circuit 123 and the other input terminal of the same NAND circuit 123 via a delay circuit 122. Therefore, two clock signals having a difference in delay time between the two are inputted to the same NAND circuit 123 as shown by (b) and (c) in FIG. 21, so that the duty ratio of an output X of the NAND circuit 123 changes according to the above-mentioned delay time as shown by (d) in FIG. 21. Further, the output X of the NAND circuit 123 is given to a buffer circuit composed of a P-channel MOS transistor M1 and an N-channel MOS transistor M2 for inversion. An output Y (pulse signal) of this buffer circuit as shown by (e) in FIG. 21 is further smoothed by a filter circuit composed of an inductor L1 and a capacitor C1, and then supplied to an integrated circuit as a driving voltage. Here, an output voltage Z (a straight line) of the filter circuit is a voltage obtained by averaging the output voltage Y of the buffer circuit, as shown by (e) in FIG. 21.
In the above-mentioned prior art voltage converter circuit, however, since the voltage supplied to the integrated circuit is a constant voltage decided according to the delay time of the delay circuit 122, that is, a constant voltage obtained in accordance with the circuit design of the voltage converter circuit, it is impossible to change the voltage supplied to the integrated circuit when the integrated circuit is operating. Therefore, there exists a problem in that an optimum driving voltage cannot be always supplied to the integrated circuit by use of the prior art voltage converter circuit.